Recently, automotive air bag systems for protecting the occupants of a vehicle have been developed. Such systems generally include a closed vessel containing compressed gas used for inflating the air bag, a vehicle deceleration responsive valve for activating a pyrotechnic system to heat the compressed gas, a means of releasing the compressed gas into the air bag upon sudden deceleration, and a pressure responsive switch (PRS) for continuously monitoring the inside pressure of the closed vessel. When the pressure level in the closed vessel is within a predetermined range, the PRS provides an electrical signal to the automobile's electrical system so that the driver can know the air bag inflation system is in a normal condition. On the other hand, when the pressure level in the closed vessel changes sufficiently so as to become too high or too low, the PRS provides a different electrical signal to the electrical system of the automobile thereby providing a failure warning to the driver.
A conventional PRS is provided with a rigid/frangible diaphragm for separating the switch from the closed vessel. This diaphragm is designed to remain intact during normal variations in pressure differential, but fail upon a discharge of gas from the pyrotechnic system, by fracturing across its face or shearing around its secured circumference, to enable flow of gas into the bag.
The switch is designed to detect a normal difference in the vessel pressure and initiate low pressure warning when either the storage vessel has leaked or the switch itself has failed by leakage. To prevent the PRS from activating in response to pressure drop as a result of only a reduction in temperature, the PRS employs temperature compensation by implementing a reference storage volume within the switch that is filled to the same pressure as the storage vessel. Since the temperature and pressure of gas inside the switch track the temperature and pressure of gas in the vessel, the switch responds to actual pressure differential as a result of leakage of gas rather than to pressure drop due to thermal contraction or expansion of the stored gas. However, upon return of pressure to normalcy, the PRS retains no record of a previously abnormal pressure condition. Because even a temporary shift in the pressure differential may indicate an incipient defect in the system, it would be desirable to require servicing of the PRS and air bag inflation system following any significant change in pressure that is not attributable to changed ambient temperature.